Mode-locking using Kerr nonlinearity, also known as Kerr-lens mode-locking (KLM), can be used to generate ultrashort pulses in the few-cycle regime directly from laser oscillators. The KLM mechanism, which is an effective saturable absorber, leads to pulse shortening; and the pulse shortening counters pulse lengthening that is caused by gain filtering experienced by the pulses. Consequently, mode-locked lasers generating femtosecond pulses are pumped well above the threshold for obtaining enough intracavity pulse energy to induce the nonlinearity and are operated on one of the edges of the cavity stability regions where the KLM strength is maximized, which usually results in less-stable operation, critical cavity alignment and reduced beam quality.
If, however, there would be no or only strongly reduced gain filtering, which implies that the total cavity loss has the same spectral profile as the gain, an arbitrary low KLM action could sustain short pulses covering the spectral range where the intracavity dispersion is well compensated. This result may lead to mode-locking with less cavity misalignment, lower intracavity pulse energy, and greatly improved beam quality. Creation of such frequency-dependent intracavity loss has been demonstrated previously by adding a thin angle-tuned etalon in a titanium-doped sapphire (Ti:sapphire) regenerative amplifier [C. P. J. Barty, et al., “Regenerative Pulse Shaping and Amplification of Ultrabroadband Optical Pulses,” 21 Opt. Lett. 219-221 (1996)]. There are several disadvantages, however, of using etalons in laser oscillators, including the following:
(1) the etalon behaves as an additional lossy component that dramatically lowers the laser efficiency;
(2) since etalons are based on multiple reflections between two fixed surfaces, the shape of the transmittivity and transmitted phase are dominated by the interference behavior and cannot be controlled independently; and
(3) for broadband lasers, the thickness of the required etalon will be very small, which makes it very challenging, in terms of alignment and manufacturing capabilities, to produce loss with the right peak location and linewidth.